1. Field of the Invention
The present invention relates to an inverter driving method for induction motors for which a wide constant power control range is needed, such as for those used for the drive of main spindles of machine tools, electric vehicles, and the like and particularly to a method in which the constant power control range is widened by changing-over a winding connection system for induction motors.
2. Prior Art
In a drive for the main spindles of machine tools and the like, connection for the motor windings are sometimes switched to a .DELTA.-Y connection for the inverter drive in order to widen the constant power control range for the machine tool, as shown in FIG. 9.
In FIG. 9, reference numeral 1 is a motor having three-phase windings 2 through 4, that is U-U', V-V', and W-W' windings, reference numeral 62 is an inverter which has a switching control function for the windings and controls the speed of motor 1, and comprises a main circuit 51 and V/F controller 63. The voltage and frequency reference VF.sub.REF is given by a frequency reference device 61. Reference numerals 6 and 7 are switches which change-over the windings to a .DELTA. connection or a Y connection. These switches 6 and 7 are changed-over by a winding change-over signal 55 sent from the controller 63 according to the winding selecting signal 22. When the switches 6 and 7 are actually changed-over, a winding change-over answer signal 54 is output to the controller 63. Reference numeral 33 is a switchgear including the switches 6 and 7. From the controller 63, a base drive signal 53 is output to the main circuit 51. This winding connection is a Y connection with the switch 6 closed and the switch 7 opened, and it is a .DELTA. connection with the switch 6 opened and the switch 7 closed. Though the configuration in FIG. 9 is for the winding connection of a Y-.DELTA. type, configurations for other winding connections are the same as above in the basic configuration.
FIG. 9 shows the case where windings of Y-.DELTA. connections are changed-over. Besides these connections, Y-2Y connection, a .DELTA.-2.DELTA. connection, and a combination of these connections are considered.
FIG. 8 represents output characteristics in both cases where the motor windings are under Y-connection and under .DELTA.-connection. In FIG. 8, symbols .circle. 1 and .circle. 2 show the bare characteristics of an induction motor of a .DELTA. connection. The curve of .circle. 1 is determined by the output current of an inverter, and the curve of 2 by the output voltage of the inverter.
Also, the line of symbol 3 is determined by the output limit of the inverter. Based on these characteristics, the induction motor is controlled so as to have a characteristic shown by the bold line. The section between a motor speed N.sub.1 and a motor speed N.sub.2 in FIG. 8 form a constant power control range. When a .DELTA. connection of motor windings is changed-over from a .DELTA. connection to a Y connection, the base speed becomes 1/.sqroot.3 times that in a .DELTA. connection, and it is in the constant power control range shown by dotted lines.
As described above, when the motor is used with its winding connection changed-over from a Y connection to a .DELTA. connection and vice versea, it is possible to control the motor in a wide constant power control range which can not be obtained by a single connection.
As an inverter drive system using such a changeover of connection, since motors having different characteristics are driven, an inverter using V/F control is generally employed.
However, as the exciting current and secondary current interfere with each other in the case of a V/F control, the dynamic characteristic of the motor is not very good.
The present invention is devised by considering such a problem, and its object is to improve the dynamic characteristic in the drive of motors provided with different characteristics depending upon the method of connection of the motor windings.